Hi,
As has already been mentioned, where you park, how many users are online and what type of card you are using are among the biggest factors that can affect your connection.
Line of sight is very important. Try to park so that you have a clear view of the restaurant/travel store. The truck-stop antennae is generally mounted atop the main building. The signal can be adversely affected by anything that interferes with line of sight. This can be obstructions in your cab or a neighboring truck. It also helps to get a parking space as close as possible to the main building.
Try to get a card that has an external antennae. That can be an internal card with an antennae or one (like I use) that connects thru a USB port and has a jumper wire that allows you to position the antennae in your cab for best reception.
Bandwidth is also important. Basically this is the amount of useable computing connection divided by the number of users. The more people using the system, the slower things will go.
All of these things, together with a few others, determine the quality of your connection. I have been extremely happy with Flying J WiFi and have signed up for a year long connection. I use it every time I have the opportunity to check my Panther II owners website to verify mileage, payroll and several other functions. Keeping track of my email is also very important to me.
Enjoy and have a safe trip,
Sevenn
Just because you are paranoid doesn't mean that no one is out to get you!